I hate the sibling twist more than anything else in SW. It feels contrived, universe-shrinking, and ultimately pointless.
As to how Vader could provoke Luke (which is actually the alleged reason for including the twist in the first place), he does say the Dark Side is the only way to save his friends. Okay, I’ll admit that’s pretty weak, but I’d let it slide to not make them siblings any day. That said, I’d certainly prefer a better cause of provocation.
How about instead of saying “twin sister” Vader says “the princess” or something like that? Then take out the dialogue of “Obi-Wan hiding her from him”, and just make the “if you won’t turn to the dark side perhaps she will” still be about Leia, but not sister Leia.
In the end I won’t mind if she’s still Luke’s sister at all though, I simply think that the idea of making her not be his sister is worth considering.